Electroactive polymers can generally be switched between two or more stable oxidation states, giving rise to changes in properties including conductivity, color, volume, and transmissivity. [G. Inzelt, M. Pineri, J. W. Schultze, and M. A. Vorotyntsev, Electrochim. Acta, 45, 2403 (2000)]. Electroactive polymers which have been oxidized from a neutral state are said to be p-doped, by analogy to semiconductor terminology. Likewise, polymers that have been reduced from a neutral state are said to be n-doped. Owing to the inherent stability of carbocations, p-dopable materials are quite well known and have been thoroughly documented. [G. Inzelt, M. Pineri, J. W. Schultze, and M. A. Vorotyntsev, Electrochim. Acta, 45, 2403 (2000); J. Jagur-Grodzinski, Polym. Adv. Tech., 13, 615 (2002); and, J. W. Schultze and H. Karabulut, Electrochim. Acta, 50 1739 (2005)]. One of the most prominent families of p-dopable polymers is that based upon polythiophene. [J. Roncali, Chem. Rev. 1992, 92, 711]. However, stable n-doped polymers have heretofore been unreported. [D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand, Synth. Met., 87, 53 (1997); K. Wilbourn and R. W. Murray, Macromolecules, 21, 89 (1988); and, M. Quinto, S. A. Jenekhe, and A. J. Bard, Chem. Mater. 13, 2824 (2001)]. Such n-doped polymers would be desirable for the same reasons that p-doped polymers have been desired and prepared, as well as for use in applications such as batteries and supercapacitors, for example. [A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, and J. P. Ferraris, J. Power Sources, 47, 89 (1994)]. The instability of n-doping conjugated polymers is most likely due to the inherent instability and highly reactive nature of carbanions as compared to carbocations. [D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand, Synth. Met., 87, 53 (1997)].
One approach being explored to obtain stable n-doping polymers is the synthesis of donor-acceptor materials. [A. Berlin, G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, and A. Zanelli, Chem. Mater., 16, 3667 (2004); D. J. Irvin, C. J. DuBois, and J. R. Reynolds, Chem. Comm. 2121 (1999); P. J. Skabara, I. M. Serebryakov, I. F. Perepichka, N. S. Sariciftci, H. Neugebauer, and A. Cravino, Macromolecules, 34, 2232 (2001); and, H—F. Lu, H. S. O. Chan, and S—C. Ng, Macromolecules, 36, 1543 (2003)]. In a donor-acceptor type of system, the polymer HOMO (highest occupied molecular orbital) is energetically similar to the relatively high-energy HOMO of the donor material, while the polymer LUMO (lowest unoccupied molecular orbital) is energetically similar to the relatively low-energy LUMO of the acceptor. This type of electronic architecture leads to a small HOMO-LUMO gap in the polymers and consequently to a low-lying polymer LUMO suitable for accepting charge.
The electron-poor functionality of the acceptor groups can be obtained in at least two ways. In the most common approach, electron-withdrawing substituents such as nitro- or fluoro-groups, including perfluorinated alkyl groups, for example, are incorporated pendant to the main chain of the polymer. [D. J. Irvin, C. J. DuBois, and J. R. Reynolds, Chem. Comm. 2121 (1999); and, P. J. Skabara, I. M. Serebryakov, I. F. Perepichka, N. S. Sariciftci, H. Neugebauer, and A. Cravino, Macromolecules, 34, 2232 (2001)]. While this method can yield electron-deficient monomer units and ultimately electron-deficient polymers, polymers with pendant electron-withdrawing groups often have resonance structures in which the charge is localized on the substituent, thus reducing carrier mobility.
Substituents can be attached to the 3- and 4-positions of the thiophene ring in order to select or to tune the properties of the material. For instance, alkyl chains have been included at the 3-position of the monomer to give the well known poly(3-alkyl thiophene) (P3AT) group [R. S. Loewe, P. C. Ewbank, J. Liu, L. Zhai, R. D. McCullough, Macromolecules 2001, 34, 4324-4333; R. S. Loewe, S. M. Kheronsky, R. D. McCullough Adv. Mater. 1999, 11, 250-253; and, R. D McCullough and P. C. Ewbank in Handbook of Conductive Polymers, (Eds: T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynolds), Marcel Dekker, Inc., New York, 1998, Chap 9]. These polymers are generally soluble in a wide array of organic solvents and generally have oxidation potentials slightly lower than that of unsubstituted polythiophene, owing to the electron-donating effect of the alkyl substituents. In order to further reduce the oxidation potential of polythiophene, stronger electron-donating groups can be attached to the monomer. The most prominent examples of this approach are the poly(alkylene dioxythiophene) or PXDOT materials such as poly(EDOT) and poly(PropOT). [L. Groenedaal, G. Zotti, P. H. Aubert, S. M. Waybright, J. R. Reynolds, Adv. Mater. 2003, 15, 855-XXX; and, J. Roncali, P. Blanchard, P. Fre're, J. Mater. Chem. 2005, 1589]. The strongly electron-donating alkoxy substituents serve to make the polymer much more electron-rich and therefore also more easily oxidized than is unsubstituted polythiophene.
The first and most common of these is the use of alternating electron-rich and electron-poor units in the polymer chain. [C. J. DuBois, K. A. Abboud, J. R. Reynolds J. Phys. Chem. B 2004, 108, 8550; D. J. Irvin, C. J. DuBois Jr., J. R. Reynolds, Chem. Commun. 1999, 2121; B.-L. Lee T. Yamamoto Macromolecules 1999, 32, 1375; C. J. DuBois, J. R. Reynolds, J. R. Adv. Mater. 2002, 14, 1844; G. Sonmez, H. B. Sonmez, C. K. F. Shen, R. W. Jost, Y. Rubin, F. Wudl Macromolecules 2005, 38, 669; and, A. Berlin, G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, A. Zanelli Chem. Mater. 2004, 16, 3667]. This arrangement provides a low HOMO-LUMO gap in the polymer and consequently a low-lying LUMO suitable for accepting charge. Another approach to producing n-doping materials is the exclusive use of electron-poor monomer units. This objective can be accomplished by attaching electron-withdrawing substituents such as nitro or perfluorinated alkyl groups to the polymer backbone. To be useful in devices such as batteries and supercapacitors, the active materials must exhibit high carrier mobility so that the devices may be discharged rapidly to provide sufficient power for practical applications. In donor-acceptor polymers, the electron-rich portions of the chain serve to reduce carrier mobility as a result of like-charge repulsion.
The present invention discloses the use of a monomer unit analogous to those of the PXDOT families to produce a new n-doping polymer, poly(3,4-difluorothiophene) (PDFT). Whereas the easily oxidized PXDOT materials feature electron-donating substituents in order to enrich the electron density of the polymer backbone, PDFT contains strongly electron-withdrawing fluorine substituents to substantially reduce the electron density of the polymer chain. This makes reduction of the material more likely. In addition, the fluorine substituents do not act as charge traps when the polymer is n-doped because no stabilizing resonance structures are available in this structure.